This invention relates generally to free-piston Stirling machines which include free-piston Stirling engines, heat pumps, coolers and cryocoolers. More particularly, the invention is directed to compact structures for attaching the cylinder, in which a piston reciprocates, to the interior of the pressurized housing for the machine. The invention compensates for distortion or movement of component parts of the housing as a result of heat or pressurization by allowing the cylinder to move with the housing component in the direction in which it needs to move with the housing component but also prevents movement and physical distortion of the cylinder as a result of housing movement or distortion in directions that, if applied to the cylinder, would distort the cylinder or otherwise harmfully damage the machine or its operation.
FIG. 1 illustrates some components of a free-piston Stirling machine having a housing 10 and a cylinder 12. Other components, such as the regenerator and a drive motor or driven load, are omitted because they are not a part of the invention and are well known in the prior art. The cylinder 12 has a cylinder flange 14, which may be an annular mounting flange, for attachment of the cylinder 12 to the housing 10. The cylinder flange 14 is attached to a housing transition plate 16 by a series of circularly spaced machine screws 18. As seen in the drawing, the transition plate 16 has a thicker metal wall for strength and resisting distortion and makes the transition between portions of the housing 10 that have different diameters. A piston 20 reciprocates in the cylinder 12 along a central axis 22 of reciprocation. Similarly, a displacer 21 also reciprocates in the cylinder 12 along the central axis 22 of reciprocation. The housing 10 is filled with a pressurized working gas in the well-known manner. The working space in the housing 10 for a Stirling engine or for a heat pump, cooler or cryocooler is located, in FIG. 1, to the left of the piston 20 and the transition plate 16. The space 23 in the housing 10 that is to the right of the piston 20 and the transition plate 16 is the back space or bounce space. This back space 23 in a Stirling engine contains the load that is driven by the engine, such as an alternator. In a heat pump, cooler or cryocooler, the back space 23 typically contains a motor, such as a linear motor, for driving the Stirling machine.
Free-piston Stirling machines typically use close-running clearance seals. That means that the clearance gap between the piston 20 and the cylinder 12 is small in order to minimize leakage through the clearance gap and to allow the effective use of gas bearings. However, free-piston Stirling machines are also subjected to extreme temperatures and to substantial temperature differences between component parts of the same machine. Additionally, in order to increase their specific power, Stirling machines are most often pressurized to a mean pressure of 3.0 MPa (450 psi) or more. Consequently, the machine and many of its component parts become physically distorted as a result of the pressurization, thermal expansion and differing thermal expansion rates of adjacent parts. The distortion of the housing 10 and of any intermediate structures connected between the housing 10 and the cylinder 12 can apply forces against the cylinder flange 14 and/or the cylinder 12 and thereby cause the cylinder 12 to move and/or distort. Because the piston to cylinder clearance gap is so small, physical distortion of the cylinder often leads to the piston rubbing against the cylinder.
FIGS. 1 and 2 illustrate a basic, prior art arrangement for attaching the cylinder 12 to a housing 10 in the manner described above. There are three directions of distortion movement that can be considered and are illustrated in FIG. 2. If the housing 10 distorts in a manner that applies a force causing the cylinder 12 and its flange 14 to move in the direction of axial translation 1 (i.e. parallel to the central axis of reciprocation 22), the cylinder 12 and its flange 14 need to move in that direction with the transition plate 16 in order to maintain the proper operation of the free-piston Stirling machine. However, in order to maintain the proper operation of the free-piston Stirling machine, if the housing, importantly including the transition plate 16, moves or distorts in the direction of radial translation 2 or in the direction of radial rotation 3, no part of the cylinder 12 or its cylinder flange 14 should be moved or distorted by that housing movement or distortion. [radial rotation 3 is the direction of movement of the transition plate 16 or the cylinder flange 14 by which radials from the central axis 22 through the transition plate 16 or the cylinder flange 14 move from being perpendicular to the central axis 22 to making an acute angle with that perpendicular.]
As an example of one form of distortion, which is quantitatively exaggerated in FIGS. 1 and 2 for visibility, when the housing 10 is pressurized, the transition plate 16 moves in a direction of radial rotation 3 to a position 16A becoming frusto-conical. With the cylinder flange 14 bolted to the transition plate 16, the cylinder flange 14 is also bent with it in a direction of radial rotation 3 to the position 14A (FIG. 2). That radial rotation 3 of the cylinder flange 14 causes the interior wall of the cylinder 12 to be distorted from a cylindrical surface to a distorted contoured surface 24. Since that occurs annularly around the cylinder 12, an axial interval of the cylinder becomes necked down; that is, pushed inward along a circular area forming an annular restriction of reduced cylinder diameter. The result is that the piston 20 would rub on that restriction. In order to avoid the rubbing, the end segment or nose of the piston can be machined to a smaller diameter to avoid the rub but that also increases seal leakage between the piston and cylinder. An alternative is to abrade or machine the interior wall of the cylinder to reduce or remove the restriction. However, this distortion is not present when the machine is depressurized and opened and moreover the area of rub is more difficult to see since this is down inside a bore. Additionally, when machining the interior of a cylinder, the cylinder wall moves out under the force of the machining tool and returns inward when the tool is removed. But the thicker part of the cylinder wall, and especially the cylinder wall directly within the cylinder flange 14, moves out less than the thinner part of the cylinder wall. Consequently the thicker part of the cylinder wall, and especially the cylinder wall directly within the cylinder flange 14, ends up with a larger diameter than the thinner parts of the cylinder wall. That variation in cylinder diameter can be alleviated or substantially reduced if the cylinder flange has circularly spaced discontinuities in its radius in the nature of castling or scallops. As can be visualized by those skilled in the art, an examination of the shape of the housing 10 reveals that pressurization and heat also cause distortion in the direction of axial translation 1 and in the direction of radial translation 2.
The goal of a cylinder mounting technique is to minimize how much the housing distortion in turn distorts the critical engine running surfaces, especially the interior of the cylinder. In particular, the cylinder 12 should be attached so that, when the housing 10 distorts, that distortion will not distort the cylinder 12. However, an additional requirement is that the cylinder 12 be relatively firmly fixed axially to the housing so the cylinder 12 does not move axially relative to the transition plate 16 when the Stirling machine is pressurized and heated or during operation when a cyclic pressure force is acting on the cross-sectional area of the cylinder.
The prior art recognized that the above-described distortion problems exist if the cylinder 12 is directly and rigidly attached to the housing 10 as illustrated in FIG. 1. One type of prior art solution has been to use an intermediate connecting member where connection length is primarily used to isolate the cylinder from the distortion of the housing. For example, a clamping cylinder can be fit coaxially over and surrounding the piston cylinder and have an outward extending flange at one end for attachment to the transition plate 16 and an inward extending flange at its opposite end for clamping against the end of the cylinder. The axial length of the clamping cylinder and a gap provided between the interior of the clamping cylinder and the exterior of the engine cylinder together allow movement of the cylinder in the radial and radial rotation directions. Although length is used to isolate both the radial rotation and radial expansion of the housing from the cylinder, these lengthy connections occupy radial space outward from the engine cylinder which then requires that the housing and a drive motor or mechanical load must have a larger diameter. Another prior art solution is to significantly thicken the engine cylinder to help reduce distortion by making it stronger. That solution, however, adds substantial weight and cost to the Stirling machine.
It is therefore an object and purpose of the invention to provide a cylinder mounting structure that attaches the cylinder to the housing so that the cylinder (1) remains stationery with respect to the housing (moves with the housing) in response to distortion in the axial direction 1, (2) is not distorted or otherwise affected by housing distortion in the direction of radial translation 2 and (3) is not distorted or otherwise affected by housing distortion in the direction of radial rotation 3.